TDMA SYSTEMS
Many digital radio transmission systems transmit data frames that comprise interspersed data and synchronization sequences. Cellular telephone systems employ such TDMA protocols and are an example of a system in which the invention hereof may be employed. Each receiving station has an assigned synchronization sequence that enables the station to selectively decode accompanying data. Such synchronization sequences are utilized in time division multiple access (TDMA) systems wherein separate users are allocated separate time slots of a same frequency bandwidth. Each time slot is accompanied by a synchronization sequence that is known to a receiving station and enables that receiving station to achieve synchronization with the transmitted signal.
Often a synchronization sequence is chosen so that it exhibits a zero autocorrelation characteristic. More specifically, if such a synchronization sequence is correlated with itself, only when the sequences being correlated are aligned does the correlator generate a pulse output. At other times, the correlator's output is zero or nearly zero. A synchronization sequence exhibiting a zero autocorrelation characteristic allows an impulse response of a channel to be estimated and enables synchronization actions to occur.
In FIG. 1, a TDMA signal train 1 indicates three identical synchronization sequences 2, 3 and 4 interspersed between a pair of data time slots. A receiver stores a copy 5 of the synchronizing sequence and causes it to be sequentially compared to the received sequences 2, 3 and 4 during autocorrelation. Only when copy 5 is perfectly aligned with a synchronizing sequence 2, 3 and 4 does an autocorrelator produce a pulse output (e.g. pulses 6, 7 and 8, respectively). Since pulse output 7 is isolated by adjacent zero outputs (due to the zero autocorrelation characteristics of the synchronizing sequence), pulse 7 can be easily isolated and used to commence a frame synchronization operation.
In FIG. 2, only a single synchronizing sequence 9 is transmitted. A partial autocorrelation can be accomplished by using only a subset of the synchronizing sequence (e.g. bits 3-10 when a synchronizing sequence 9 comprises bits 1-12). During times A and C, the subset shows a random correlation with incoming data. During time B a partial autocorrelation function is exhibited, and during time D, a zero autocorrelation property is exhibited.
In FIGS. 1 and 2, a symbol sample rate of once per symbol is assumed. Under such circumstances, frame synchronization, plus or minus one symbol time, can be achieved. In FIG. 3, an autocorrelation function is shown that is achieved when the sample rate is performed at twice the symbol rate. Such a partial autocorrelation takes the approximate form of the impulse response of the channel.
Cellular telephone systems often suffer from multipath propagation effects, where a receiver sees copies of a transmitted signal that have traveled different paths between the transmitter and the receiver. Generally such paths are of different lengths and cause these copies of the transmitted signals to be delayed relative to each other. If the signalling rates in such systems are sufficiently high, multipath propagation causes intersymbol interference which, in turn, makes signal detection impossible.
The term symbol is used in this context to refer to transmitted signals that are phase modulated with discrete phase relationships, each assigned phase relationship being a symbol that is subject to detection at a receiver. The term intersymbol interference refers to two or more symbols that are superimposed upon each other, phase detection of each symbol thus becoming extremely difficult, if not impossible.
Systems for modulating the phase of a carrier wave to represent digital binary data are known in the prior art. A four state modulator (quaternary phase shift keying or QPSK) enables a carrier wave to take four different phase values depending on values assumed by successive two-bit binary groups. Each of the four equi-spaced phases is separated by 90.degree..
Generally, if no pilot synchronization signal is generated by the transmitter, the receiver must derive symbol timing from the received signal. Both the transmitter and receiver employ separate station clocks which drift, relative to each other, and any symbol synchronization technique must be able to track such drift. Furthermore, in the case of intersymbol interference as a result of multi-path propagation, many receivers employ equalization techniques to enable differentiation of multi-path received signals. If the received signal is to be detected without aid of an equalization procedure, the time of sampling of the received signal must be optimally chosen. If equalization is used, the method chosen must be able to operate over a time span at least as long as the delay between the signal paths. However, the longer the time span of the equalizing method, the more computation power is required. To minimize the required computation power, the equalizing method needs to be optimally aligned to the incoming signal. The problem is further complicated by the fact that where one of the communicating stations is mobile, the multi-path propagation phenomena is accompanied by signal jitter during the reception window.